Measuring the properties of nothin?

Physicists start measuring the properties of vacuum.

In the old days life was easy.On Earth there was stuff, on other planets and stars there was stuff, but in between there was a whole lot of nothing. The vacuum did nothing except, perhaps, maintain our perspective as a pretty insignificant bunch of creatures.Then Paul Dirac turned up and adjusted quantum mechanics to include special relativity, which allowed it to make accurate predictions of high energy/velocity particles.In doing so Dirac predicted the existence of the anti-electron and the presence of “virtual states” in the vacuum.Later, Feynman and others integrated quantum mechanics with electromagentism, which finally killed any prospect of the vacuum being nothing.The combination of the two means that a real particle in the vacuum can interact with virtual particles that exist there, which can have the effect of making those particles temporarily real.Thus, the vacuum teems with life of the virtual particle kind, and the vacuum is no longer than inert place it was once thought to be.

Even though the vacuum is not inert, its interactions are incredibly weak so measuring its properties is difficult.One of the predicted effects is that the vacuum combined with a strong magnetic field should rotate the polarization of light.This effect is exactly the same as happens with some crystals.Upon application of a strong magnetic field, light propagating through the crystal will have its polarization rotated.Researchers in Italy have managed to measure this rotation due to vacuum in a very clever way.Since the effective rotation per meter is very tiny, they placed two mirrors facing each other so that light can reflect between them (called an optical cavity).The optical cavity was placed inside an evacuated chamber.A strong magnetic field (5 Tesla) was then applied and light shone into the cavity from a laser.The mirrors are so highly reflecting that a single photon can be expected to pass through the magnetic field 460,000 times, thus the effective distance traveled in the magnetic field is about 2760 km.They compared the polarization of light exiting the cavity to the polarization of the laser and it was found to have rotated about 12 microradians, confirming that the vacuum is indeed optically active.

Another nice aspect of this experiment is that it is a relatively cheap and inexpensive way to infer the existence of some particles.For instance, the observed rotation is mostly likely due to the interaction between the photons and a light Boson with a fairly well defined mass.Expect further experiments to elucidate other vacuum properties and to constrain the masses of other vacuum virtual particles.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.